This invention relates to the bioconversion of a substrate by bacterial fermentation, and more particularly, to a continuous process for the production of ethanol by fermentation using strains of Zymomonas bacteria.
Bacterial ethanol fermentation has been known in the art for many years, and in recent years fermentation using strains of Zymomonas mobilis has received increasing attention. The Z. mobilis strains convert a suitable substrate, such as glucose or another sugar, to ethanol. Significantly higher specific rates of sugar uptake and ethanol production and improved yield compared to traditional yeast fermentation have been reported for these Zymomonas strains.
Fermentation by Z. mobilis has been carried out in batch and continuous culture. The fermentation product (ethanol) is dissolved in the liquid medium in the fermenter. The liquid medium is separated from solids (chiefly biomass) before the ethanol is recovered. Separation of these two phases early in the product-recovery process train is required. Before such a fermentation achieves commercial acceptance, however, productivity must be improved. The reported efforts to date have focused on developing more productive bacterial strains and modifying the configuration of the fermenter used in the fermentation. For example, improvement in ethanol productivity using a continuous culture with a cell recycle system has been reported with Z. mobilis strains.
Recovery of the fermentation product can be a complex and multifaceted task. A significant proportion of the overall cost in fermentation plant often must be spent for ethanol recovery. A recently reported technique involves the use of a flocculent strain of Z. mobilis that settles in the fermenter allowing the supernatant containing the ethanol to be withdrawn while leaving a majority of the cells in the fermenter. This method is based on the well-known gravity sedimentation principle for separating liquids and solids. The more conventional approaches for separating fermentation broth from biomass involve the withdrawal of a portion of the culture medium from the fermenter and separation of the two phases by centrifugation or filtration techniques. Regardless of the technique employed, for a particular ethanol recovery process, it is desirable to reduce the quantity of biomass in order to reduce the load of solids on the sedimentation, centrifugation or filtration apparatus.
At the same time, however, the yield of ethanol from the fermentation must be maximized. Since product formation cannot occur in the absence of biomass, ethanol formation is dependent on cell mass concentration. In fact, the rate of ethanol production in the fermenter is directly proportional to the quantity of biomass in the fermenter under steady state conditions. Thus, within the limits of the metabolic regulatory controls of the microorganism and process dynamics, increasing the biomass in the fermenter while maintaining other conditions constant will shorten the time required to produce a given amount of ethanol. However, this seemingly simple approach for optimizing process performance will have an adverse effect on the ethanol recovery process because the load of solids on the separating equipment will be correspondingly increased.
It is well known that the substrate, such as glucose, is the largest item of raw material cost in the fermentation. Therefore, the presence of substrate in the effluent from the fermenter in a continuous fermentation is to be avoided. The continuous fermentation should be conducted at optimum process product yield, which occurs when the substrate is completely converted to ethanol and when the substrate is minimally diverted from product (ethanol) formation to cell mass synthesis (i.e., when the growth yield with respect to carbon substrate is minimized).
Thus, there exists a need in the art of a continuous process for the production of ethanol using strains of Zymomonas in which the substrate fed to the fermenter is converted to ethanol in as short a time as possible. The process should permit a reduction in the quantity of biomass in the fermenter in order to obtain a corresponding reduction in the load of solids on the ethanol recovery apparatus. In addition, the quantity of substrate in the effluent from the fermenter should be minimized.